Issues with two Haskell commands, one attempting to find and print differences in two lists, and one attempting to output the longest length

Question

Here is the first one, which is attempting to take two lists and print all the differences between the two .

EX.

> list_diff [1,6,1,3,4,3,5] [3,8,5,4,7,4,8] [1,6,1,8,7,8]

list_diff :: Eq a => [a] -> [a] -> [a]
list_diff [] [] = []
list_diff x y = list_helper x y

list_helper [] [] = []
list_helper (x:xs) y
     | x `elem` y = list_helper xs y
     | otherwise = x:(list_helper xs y)

When I run a test for this one (using the example provided above), I reviece the following output:

[1,6,1*** Exception: HW1.hs:(12,1)-(15,39): Non-exhaustive patterns in function list_helper

For the second funtion, I am attempting to take a series of values and determine which list is the longest

EX.

progLanguages = 
     [ ("CptS121" , ["C"]), 
     ("CptS122" , ["C++"]), 
     ("CptS223" , ["C++"]), 
     ("CptS233" , ["Java"]), 
     ("CptS321" , ["C#"]), 
     ("CptS322" , ["Python", "JavaScript"])]

INPUT: max_count progLanguages

OUTPUT: ("CptS322" , 2)

What I have so far isn't working at all, so needless to say I am stuck.

max_count [] = error "bad"
max_count [x] = x
max_count (x:xs) = x max_helper (max_count xs)
     where
     max_helper (a,b) (a',b')
          | length b > length b' = (a, length b)
          | otherwise = (a', length b')

Thanks in advance for any help you can give.

How does `list_helper` handle its first argument being an empty list and the second argument _not_ being empty? — Chris, Feb 09 '22 at 22:09
Turn warnings on with `-Wall` to detect these mistakes at compile time instead of runtime. The warning error will also tell you the cases you missed. (IMO, this should be a hard error by default.) — chi, Feb 09 '22 at 22:40

Chris · Accepted Answer · 2022-02-10T04:28:02.323

You've defined list_helper:

list_helper [] [] = []
list_helper (x:xs) y
     | x `elem` y = list_helper xs y
     | otherwise = x:(list_helper xs y)

The warning about non-exhaustive pattern matching means that your function is evaluating arguments that you haven't told it how to handle. In this case, that's the first argument being an empty list, but the second argument not being empty.

You need to handle this pattern:

list_helper [] [] = []
list_helper [] y = y
list_helper (x:xs) y
     | x `elem` y = list_helper xs y
     | otherwise = x:(list_helper xs y)

Prelude> :{
Prelude| list_helper [] [] = []
Prelude| list_helper [] y = y
Prelude| list_helper (x:xs) y
Prelude|      | x `elem` y = list_helper xs y
Prelude|      | otherwise = x:(list_helper xs y)
Prelude| :}
Prelude> list_helper [1, 2, 3] [4, 5, 6]
[1,2,3,4,5,6]

Alright, thanks! It's working now! – J.Bauman Feb 09 '22 at 22:25 — J.Bauman, Feb 09 '22 at 22:25

Issues with two Haskell commands, one attempting to find and print differences in two lists, and one attempting to output the longest length

1 Answers1